Method and apparatus for use with an electron gun employing a thermionic source of electrons

ABSTRACT

A method includes providing an electron gun having a first head with a thermionic electron source and an accelerating electrode, and further includes replacing the first head with a second head having a power rating substantially different than that of the first head, and subsequently operating the electron gun without replacing the accelerating electrode. The electron gun may further include a platform spaced apart from the accelerating electrode and having an adjustably located locating member that engages a reference member on the head to position the head in three dimensions relative to the accelerating electrode. The platform may be adjustably spaced from the accelerating electrode in order vary the distance between the electron source and the accelerating electrode.

This is a division of copending application Ser. No. 09/210,632, filedon Dec. 11,1998.

RELATED APPLICATION

The subject matter herein may be disclosed and/or claimed in U.S. patentapplication Ser. No. 09/209,629 entitled “APPARATUS FOR AN ELECTRON GUNEMPLOYING A THERMIONIC ELECTRON SOURCE”.

TECHNICAL FIELD

This invention relates to an electron gun and more particularly to anelectron gun of a type having a thermionic source of electrons disposedon a head, an accelerating electrode, and a platform to support andposition the head relative to the accelerating electrode.

BACKGROUND

Electron beam furnaces are used to heat materials to produce vapors fordeposition on an article. An electron beam furnace includes an electrongun, a deflection system, and a cooling system. The electron gungenerates an electron bean. The deflection system directs the electronbeam toward the material to be heated. The cooling system cools theelectron gun to prevent it from overheating.

The electron gun typically includes an electron source, a focusingelectrode, and an accelerating electrode. The electron source istypically a cathode heated by an electric current to cause the cathodeto emit electrons. The focusing electrode is typically negativelycharged to repel the electrons and thereby direct the electrons in adirection generally toward the accelerating electrode. The acceleratingelectrode is positioned downstream from the electron source and thefocusing electrode. The accelerating electrode is typically lessnegatively charged than the electron source and the focusing electrodeto cause the electrons to form into a beam and travel in the downstreamdirection.

In one known type of electron gun, the electron source and the focusingelectrode are elongated and disposed in a head. The head is supported bya platform spaced apart from the accelerating electrode. This type ofelectron gun is reliable and available in many different power ratings.The physical size of the head, the accelerating electrode, and theplatform of a given one of these electron guns depends on its powerrating.

It is important that an electron gun to be used in an electron beamfurnace generate an electron beam suitable for the type of material tobe heated and the type of deposition sought for the article. Differenttypes of materials and depositions require electron beams of differentamounts of electron beam power and may require different electron beamshape. However, it is desirable to have electron beam guns operate in aspace charge limited mode. In such mode, the above described type ofelectron guns generally perform best at a power equal to or slightlybelow its power rating. Consequently, no one of these electron guns canadequately generate electron beams for all of the electron beam powersrequired.

Whenever an electron gun in the electron beam furnace does not generatean electron beam suitable for the type of material to be heated and thetype of deposition sought for the article, the traditional approach hasbeen to remove the electron gun and replace it with an electron gun thatprovides a suitable electron beam. However completely replacing anelectron gun can be difficult and time consuming. Moreover, thedeflection system and the cooling system of the electron beam furnaceare connected to the accelerating electrode of the electron gun and aredesigned to suit the physical size of the accelerating electrode and thepower rating of the electron gun. Consequently, replacing the electrongun requires replacing the deflection system and the cooling system,thereby compounding effort involved.

SUMMARY OF THE INVENTION

An object of the present invention is to reduce the effort involved withproviding an electron gun of the above described type to generate asuitable electron beam for a particular type of material and deposition.

The present invention is predicated in part on the recognition that theaccelerating electrode of a first electron gun having a first powerrating can be operated with a head from a second electron gun having asecond power rating substantially different than the first power rating,to provide an electron beam comparable to that which would be providedby the second electron gun, and that such operation is facilitated bymaking the platform adjustable enough to be able to support and positionthe head of the second electron gun, which may be physically smallerthan the head of the first electron gun.

According to a first aspect of the present invention, a method includesproviding an electron gun having an accelerating electrode and a firsthead with a thermionic electron source, and further includes replacingthe first head with a second head having a power rating substantiallydifferent than that of the first head, and subsequently operating theelectron gun without replacing the accelerating electrode.

This method reduces the effort associated with providing an electron gunthat generates an electron beam suitable for a particular type ofmaterial and deposition. As used herein substantially different meansthat one of the heads is at least twenty five percent less than theother of the heads. Using this method, suitable electron beams ofvarious power levels can be generated by replacing the head of anelectron gun without the need to replacing the accelerating electrode ofthe electron gun, thereby saving time. In one detailed embodiment, thepower rating of the second head is at least twenty five percent lessthan the power rating of the first head. In another detailed embodiment,the accelerating electrode is connected to a deflection system and acooling system, at least one of the deflection system and the coolingsystem is not replaced prior to operating the electron gun with thesecond head, thereby reducing the difficulty and the amount of timeconsumed.

According to a second aspect of the present invention, an apparatus foran electron gun has a head having a thermionic electron source and atleast one reference member, an accelerating electrode, and a platformspaced apart from the accelerating electrode and having at least onelocating member that engages the at least one reference member of thehead to position the head in three dimensions relative to theaccelerating electrode, wherein the at least one locating member isadjustably located and the location of the at least one locating membercan be adjusted by at least nine millimeters (mm).

Such apparatus is useful in practicing the above described method, butis not limited to such. In order to operate the accelerating electrodeof the first electron gun with the head of the second electron gun, itis desirable to be able to support and position the head on the platformof the first electron gun. However, as described above, the head of thesecond electron gun may be smaller in size than the head of the firstelectron gun. Providing the platform with locating members that areadjustably located by at least nine mm enables the platform to supportand position heads of various sizes.

As described hereinbelow, although adjustably located locating membersare known, until now, their adjustability was limited to less than fivemm, being merely intended to compensate for manufacturing tolerances ofcomponents of the electron gun and to facilitate alignment of theelectron source and the accelerating electrode.

According to a third aspect of the present invention, an apparatus foran electron gun has a head having a thermionic electron source, anaccelerating electrode, and a platform that supports the head and isadjustably spaced from the accelerating electrode.

This apparatus enables the shape of the electron beam to be varied byvarying the distance between the electron source and the acceleratingelectrode. The apparatus is useful in practicing the above describedmethod, but is not limited to such.

These and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description, accompanying drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, partially exploded, partially cut away view ofan electron gun;

FIG. 2 is perspective view of a thermionic electron source in accordancewith one embodiment of the present invention for use in the electron gunof FIG. 1;

FIG. 3 is a cross section view in the direction of 3—3 of FIG. 2, of thethermionic electron source of FIG. 2;

FIG. 4 is a cross section view in the direction of 4—4 of FIG. 2, of thethermionic electron source of FIG. 2;

FIG. 5 is a cross section view in the direction of 5—5 of FIG. 2, of thethermionic electron source of FIG. 2;

FIG. 6 is a cross section view in the direction of 6—6 of FIG. 1, of thefocusing electrode and the thermionic electron source used in theelectron gun of FIG. 1;

FIG. 7 is a side view of a prior art screw and a top view of a prior artspacer with an elongated hole;

FIG. 8 is a side view of a screw and a top view of a spacer of theelectron gun of FIG. 1;

FIG. 9 is a graph of a power density distribution of an electron beamresulting from a prior art thermionic electron source; and

FIG. 10 is a graph of a power density distribution of an electron beamresulting from the thermionic electron source of FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is disclosed herein with respect to a best modeembodiment for use in an electron gun illustrated in FIG. 1. Referringnow to FIG. 1, an electron gun 20 for an electron beam furnace (notshown) has an accelerating electrode 22, a platform 24, and a head 26.The electron gun 20 has a power rating of about sixty five kilowatts.The electron gun 20 is representative of in shape, but larger, thanelectron guns (not shown) having a power rating less than sixty fivekilowatts, e.g., forty five kilowatts. The accelerating electrode 22 ofthe electron gun 20 has a plate portion 28 and a beam shaper portion 30.The beam shaper portion 30 is elongated and extends in a longitudinaldirection L. The accelerating electrode 22 further has an elongatedaperture 32 that extends in the longitudinal direction L and provides apath for electrons, generated by the head 26, to exit the electron gun20. The accelerating electrode 22 may comprise a copper material and maybe formed as one piece for example by milling. Alternatively, theaccelerating electrode 22 may be an assembly wherein the plate portion28 comprises a stainless steel material and the beam shaper portion 30comprises a copper material.

A plurality of bolts 34 connects the accelerating electrode 22 to acooling plate 36 and a deflection system 38. Washer plates 40 recessedin the accelerating electrode 22 help distribute the load applied by thebolts 34. The cooling plate 36 is part of a cooling system, representedin part by a pair of water pipes 42. The cooling system 42 and thedeflection system 38 are specifically designed to suit the physical sizeof the accelerating electrode 22 and the power rating of the electrongun 20. For example, the cooling plate 36 is sized to contact as much ofthe surface area of the accelerating electrode 22 as is possible withoutcreating interference to other structures on the accelerating electrode22. This maximizes heat transfer between the accelerating electrode 22and the cooling system 42 and thereby helps prevent the electron gun 20from overheating. The cooling plate 36 and the deflection system 38 areillustrative of but physically larger than a cooling plate and adeflection system respectively suited to the physical size and the powerrating of an electron gun having a power rating of forty five kilowatts.Consequently, the cooling plate 36 and the deflection system 38 are toolarge to connect to the accelerating electrode of an electron gun havinga power rating of forty five kilowatts. The accelerating electrode 22may be electrically grounded by way of its connection to the coolingplate 36 and the deflection system 38.

The accelerating electrode 22 supports a first platform support 44 and asecond platform support 46. The first platform support 44 comprises ahigh voltage insulator 50 and an insulator cover 52. The high voltageinsulator 50, which may comprise a ceramic material, has a first end 54and a second end 56. The first end 54 has a threaded stud 58 thatextends through a washer 60 and the accelerating electrode 22 andengages a nut 62 to retain the high voltage insulator 50 to theaccelerating electrode 22. The second end 56 of the high voltageinsulator 50 has a shoulder 64 and a threaded stud 66. The shoulder 64abuts a collar 68 on the insulator cover 52. The threaded stud 66extends through the insulator cover 52 and engages a threaded cap 70 toretain the high voltage insulator 50 to the insulator cover 52. Theinsulator cover 52 limits formation of deposits on the high voltageinsulator 50. The insulator cover 52 further includes a threadedengagement surface 71. The second platform support 46 comprises a highvoltage insulator 72, an insulator cover 73, and an insulator cover 74,which are identical to the high voltage insulator 50, the insulatorcover 52, and the threaded cap 70 of the first platform support 44,respectively.

The first platform support 44 and the second platform support 46 eachfurther comprise a support ring, represented by a support ring 75. Thesupport rings 75 each have a threaded engagement surface (not shown) anda support surface, represented by a support surface 76. The threadedengagement surfaces of the support rings 75 engage the threadedengagement surfaces of the insulator covers 52, 73 to retain the supportrings 75 to the insulator covers 52, 73. The support surfaces 76 of thesupport rings 75 provide support for the platform 24 and space theplatform 24 apart from the accelerating electrode 22. Retaining rings 78engage the insulator covers 52, 73 and retain the platform 24 on thesupport surfaces 76 of the support rings 75. Adjusting the spacingbetween one of the support surfaces 76 and the accelerating electrode 22is accomplished by loosening one of the support ring 75 and theretaining ring 78, and subsequently tightening the other of the supportring 75 and the retaining ring 78. Adjusting the spacing between one ormore of the support surfaces 76 and the accelerating electrode 22 ineffect adjusts the spacing between the platform 24 and the acceleratingelectrode 22.

The platform 24, which supports the head 26 of the electron gun 40, hasan opening 80. The platform 24 further has a first locating member 82and a second locating member 84 disposed on opposite sides of theopening 80. The first locating member 82 comprises a spacer 86 and aprojection 88 extending therefrom. The second locating member 84comprises a spacer 90 and a projection 92 extending therefrom. Each ofthe projections may have a notch, represented by a notch 94, which arepart of a detent mechanism described hereinbelow. The spacer 86 of thefirst locating member 82 and the spacer 90 of the second locating member84 each have two holes, represented by a hole 96. Screws 98 extendthrough the holes 96 and engage the platform 24 to retain the firstlocating member 82 and the second locating member 84 to the platform 24.The screws 98 each have a head and a shank, represented by a shank 100.The shank 100 has a diameter 102. The holes 96 are preferably elongatedrelative to the diameter 102 of the shank 100 in a widthwise directionW, transverse to the longitudinal direction L, to provide a clearancebetween the spacers 86, 90 and the shank 100 of the screws 98. Thisclearance facilitates adjustment of the position of the locating memberrelative to the platform 24 and the accelerating electrode 22. Ifdesired, the holes 96 may be elongated in more than one directionrelative to the shank diameter 102. However, elongating the holes inonly one direction relative to the shank diameter helps to preventmisalignment between a thermionic electron source, describedhereinbelow, and the elongated aperture 32 of the accelerating electrode22. Repositioning of the first locating member 82 or the second locatingmember 84 is accomplished by loosening one or both of the screws 98 thatretain the locating member to the platform 24, positioning the locatingmember, and subsequently re-tightening the screws 98.

Referring now to FIG. 7, a prior art locating member 104A has a spacer104B with a hole 104C that is elongated. The hole 104C has a dimension104D of less than ten mm in the widthwise direction W. The hole 104C hasa dimension 104F of about 6.5 mm in the longitudinal direction L. Ascrew 104H employed in connection with the hole 104C to position thelocating member 104A has a head and a shank. The shank is threaded andhas a diameter 104I of five mm. The dimensions of the screw 104H and thehole 104C result in clearance between the spacer 104B and the screw 104Hand thereby result in adjustability of the locating member 104A. Theadjustability in the widthwise direction W is less than five mm (lessthan 10 mm−5mm). The adjustability in the longitudinal direction L isabout 1.5 mm (about 6.5 mm−5 mm). The adjustability in the widthwisedirection W and the adjustability in the lengthwise direction L areintended to compensate for manufacturing tolerances of components of theprior art electron gun (not shown) and to facilitate alignment of theelectron source and the accelerating electrode in the prior art electrongun Consequently, as will be understood in view of the discussionhereinbelow, there is less than desired adjustability with the prior artlocating member 104A to position a head from an electron gun that has apower rating of forty five kilowatts, which is physically smaller thanthe head 26 of the electron gun 20.

Referring now to FIG. 8, in one embodiment of the present invention, thehole 96 has a dimension 106 at least fifteen mm in the widthwisedirection W. The hole 96 has a dimension 108 of six mm in the lengthwisedirection L. The shank 100 of the screw 98 has a portion with a collarand a portion that is threaded. The portion with the collar has adiameter 110 of 5.95 mm and a dimension 111 that is less than thethickness of the spacer 90. The portion that is threaded has a dimension109 of 5 mm to engage the platform 24 (FIG. 1). The dimensions of thescrew 98 and the hole 96 result in clearance between the spacer 90 andthe screw 98 and thereby adjustability of the locating member 84. Theadjustability in the widthwise direction W is at least nine mm (15 mm−6mm). The adjustability in the lengthwise direction L is 0.05 mm (6mm−5.95 mm). As will be evident in view of the discussion hereinbelow,the adjustability in this embodiment is enough to position the head 26of the electron gun 20, and enough to position a head from an electrongun having a power rating of forty five kilowatts.

Referring again to FIG. 1, the head 26 includes a frame member 112 thatis U shaped and comprises a stainless steel material. The frame member112 has a first side wall 113 and a second side wall 114. The first sidewall 113 has a first reference member 116 having the shape of a recess.The second side wall 114 has a second reference member 118 having theshape of a recess. The first reference member 116 and the secondreference member 118 have a distance D between them. The distance Ddepends on the size of the head 26, which in turn depends on the powerrating of the electron gun 20. For the sixty five kilowatt electron gun20, the distance D is fifty mm, center to center. Note that for a fortyfive kilowatt electron gun, the distance between reference members isforty five mm, which is five mm (50 mm−45 mm) less than that of theelectron gun 20. When the frame member 112 is placed on the platform 24,the first reference member 116 and the second reference member 118engage the first locating member 82 of the platform 24 and the secondlocating member 84 of the platform 24, respectively, to position thehead 26 on the platform 24 and thereby positioning the head 26 in threedimensions relative to the accelerating electrode 22.

The frame member 112 may further have a pair of catch assemblies,represented by a catch assembly 122. Each catch assembly 122 has a ball124 and a spring 126. The catch assemblies 122 cooperate with thenotches 94 in the projections 88, 92 to define a detent mechanism thatretains the head 26 to the platform 24. For example, the ball 124engages the notch 94 in the projection 92 of the second locating member84. The spring 126 biases the ball 124 toward the notch 94. A pair ofscrews, represented by a screw 128, adjusts the bias provided by thesprings 126.

The head 26 further includes a first terminal 130, a second terminal132, and a thermionic cathode assembly 134. The first terminal 130engages the frame member 112. The second terminal 132 engages aconductor 136. The conductor 136 mechanically and electrically connectsthe second terminal 132 to a pair of non-magnetic, spring conductors 138that extend through the opening 80 of the platform 24 and support thethermionic cathode assembly 134. A plurality of screws 140 connects theconductor 136 to an insulator 142. Wedge shaped members 144 clamp theinsulator 142 to the frame member 112. A plurality of screws 146 biasesthe wedge shaped members 144 toward the insulator 142.

The thermionic cathode assembly 134 includes a thermionic electronsource 150 and a focusing electrode 152. The thermionic electron source150 and the focusing electrode 152 are spaced apart from one another andeach extends in the longitudinal direction L. The thermionic electronsource 150 is one piece and may comprise a tungsten material. Thefocusing electrode 152 extends partially around the thermionic electronsource 150 along a portion of a length 154 of the thermionic electronsource 150. The focusing electrode 152 has a notch 156 that extends inthe longitudinal direction L. The notch 156 is bordered by a recessedsurface 158. An ion trap 160 extends longitudinally and into the notch156 so as to be between the recessed surface 158 of the focusingelectrode 152 and the thermionic electron source 150. The ion trap 160is sacrificial in that it is expected that the ion trap 160 will bebombarded by ions and erode over time. The ion trap 160 reduces theamount of bombardment and erosion experienced by the focusing electrode152. The ion trap 160 is less costly to replace than the focusingelectrode and may comprise a carbon material. Note that the opening 80of the platform 24 is large enough for the thermionic cathode assembly134 to pass through so as to facilitate positioning the thermioniccathode assembly 134 proximate to the accelerating electrode 22.

The head 26 further includes a first holder 162 and a second holder 164.The first holder 162 is mechanically and electrically connected to thefocusing electrode 152 by fasteners 166. The first holder 162 has aclamping plate 168 and a screw 170. The screw 170 engages the clampingplate 168 and thereby causes it to tightly engage the thermionicelectron source 150. The second holder 164 is mechanically andelectrically connected to the frame member 112 by fasteners 172. Thesecond holder 164 has a clamping plate 174 and a screw 176. The screw176 engages the clamping plate 174 and thereby causes it to tightlyengage the thermionic electron source 150.

Referring now to FIGS. 2-5, the thermionic electron source has a firstend portion 180, a second end portion 182, and an aperture disposedtherebetween 184. The aperture 184 extends a portion of the length 154and a portion of a width 186 of the thermionic electron source 150. Thethermionic electron source 150 further comprises a first longitudinalportion 190 and a second longitudinal portion 192 that extend in thelongitudinal direction L and are spaced apart from one another by theaperture 184. The first end portion 180 and the second end portion 182rigidly join the first longitudinal portion 190 and the secondlongitudinal portion 192 together. The thermionic electron source may begenerally uniform in thickness 194. As illustrated, the aperture 184diminishes in width (i.e., tapers) near the ends of the firstlongitudinal portion 190 and the second longitudinal portion 192,although the aperture is not limited to such. The tapering helps toreduce buildup of stress and aids fabrication of the thermionic electronsource.

Referring also now to FIG. 6, the first longitudinal portion 190 has asurface 196 that opposes the accelerating electrode 22; the secondlongitudinal portion 192 has a surface 198 that opposes the acceleratingelectrode 22. The surface 196 and the surface 198 may be inclined andface toward each other. Due to the incline, the thermionic electronsource 150 may have a widthwise cross section having the shape of achevron, as illustrated in FIG. 4. The incline of the surfaces 196, 198may diminish near the ends of the longitudinal portions in an effort tominimize stress. Making the surfaces 196, 198 inclined rather thancoplanar with each other is a way to increase to the width of theaperture without decreasing the surface area of the surfaces 196, 198.Note that the power rating of the electron gun 20 is related to thesurface area of the surfaces that face toward the accelerating electrode22. Depending on the incline and the width of the aperture 184, thethermionic electron source 150 may have almost as much surface areafacing toward the accelerating electrode 22 as the thermionic electronsource 20 would have in the absence of the aperture 184. The thermionicelectron source 150 and the ion trap 160 are preferably aligned with theelongated aperture 32 of the accelerating electrode 22, to maximize thebenefit of the aperture and the ion trap described below.

The thermionic electron source 150 may be fabricated using any suitablemethod including but not limited to pressing, rolling, and machining(including but not limited to electrical discharge machining and lasermachining) and combinations thereof. The thermionic electron source 150may be fabricated from a thermionic electron source that does not havean aperture 184 and has been used in the electron gun 20 and undergoneion bombardment.

For the electron gun 20, which has a power rating sixty five kilowatts,the thermionic electron source 150 has a length 154 of one hundred mmand a width 186 of about three mm. The focusing electrode 152 (FIGS. 1,6) has a length of sixty five mm and a width of about thirty two mm.There is clearance 200 of about 0.5 millimeter between the thermionicelectron source 150 and the focusing electrode 152. The sacrificial iontrap 160 (FIGS. 1, 6) has a length equal to that of the focusingelectrode 152 and has a width in a range of about 1.5 mm to about twomm. The length of the aperture 184 is about sixty mm, which is about tenpercent less than the sixty five millimeter length of the focusingelectrode 152. The width of the aperture 184 is about 0.75 mm, which isabout one quarter of the width 186 of the thermionic electron source150. Note that the length 154 of the thermionic electron source 150 andthe length of the focusing electrode 152 typically depend on the powerrating of the electron gun, but the width 186 of the thermionic electronsource and the width of the focusing electrode 152 typical do not dependon the power rating of the electron gun. For example, for an electrongun having a power rating of forty five kilowatts, the thermionicelectron source 150 has a length 154 of eighty mm and a width 186 ofabout three mm. The focusing electrode 152 has a length of forty five mmand a width of about thirty two mm.

In operation, the first terminal 130 and the second terminal 132 areconnected to a power supply (not shown). The power supply provides asource of electric current for the thermionic electron source 150. Theelectric current flows through the first terminal 130, the second holder164, the thermionic electron source 150, the first holder 162, thefocusing electrode 152, the spring conductors 138, the conductor 136,and the second terminal 132. As the electric current flows through thethermionic electron source 150 it results in heating thereof, to arelatively high temperature, but typically below the melting temperatureof tungsten, causing the thermionic electron source 150 to emitelectrons. The voltage across the thermionic electron source 150 istypically less than ten volts. Because the heating for the thermionicelectron source results from an electric current, the electron gun isreferred to as directly heated. A second power (not shown) provides thesecond terminal 132 with a negative voltage potential (typically about−20 kilovolts), which is in turn provided to the first holder 162 andthe focusing electrode 152 through the conductor 136 and the springconductor 138. The negative voltage potential causes the focusingelectrode 152 to repel the electrons and thereby direct the electrons ina direction generally toward the accelerating electrode 22. Theaccelerating electrode 22 is typically at an electrical ground voltagepotential by way of the connection between the accelerating electrode22, the cooling system 42, and the deflection system 38. Theaccelerating electrode causes the electrons to form into a beam andtravel in the downstream direction. The electron beam exits the electrongun 20 through the elongated aperture 32 of the accelerating electrode22.

The electron beam from the electron gun typically extends in thelongitudinal direction L and the widthwise direction W, and has agenerally rectangular cross section in a plane containing thelongitudinal direction L and the widthwise direction W. The electronbeam has a power density that varies across its width (i.e., in thewidthwise direction W). Referring now to FIG. 9, a graph illustrates apower density distribution obtained from the electron gun with a priorart thermionic electron source. The power density distribution hascharacteristics similar to that of a Gaussian distribution. Referringnow to FIG. 10, a graph illustrates a power density distributionobtained from the electron gun 20 with the thermionic electron source150. The power density distribution has characteristics similar to thatof a Gaussian distribution, but with some variation due to the aperture184 of the thermionic electron source.

The shape and the power density distribution of the electron beamdepends on the distance between the thermionic electron source 150 andthe accelerating electrode 22, and also depends on the differencebetween the voltage potential of the thermionic electron source 150 andthe voltage potential of the accelerating electrode 22. However, becausethe platform 24 is adjustably spaced from the accelerating electrode 22,various electron beam shapes and various power density distributions maybe obtained by moving the platform 24 closer to or farther away from theaccelerating electrode 22, without the need to vary the voltagepotential between the thermionic electron source and the acceleratingelectrode. There is preferably at least one inch of adjustability tomake possible a wide range of electron beam shapes and power densitydistributions.

The electron beam is used in the electron beam furnace (not shown) tovaporize materials for deposition on articles. Positively charged ionsare produced as a result of the vaporization and of the material in theelectron beam furnace. Some of these ions have a direction of travelopposite that of the electrons in the electron beam causing the ions totravel through the elongated aperture in the accelerating electrode andtoward the thermionic electron source. The ions have the potential tobombard and erode the thermionic electron source.

However, because the thermionic electron source has an aperture, many ofthese positively charged ions do not strike the thermionic electronsource 150, but rather pass through the aperture and strike thesacrificial ion trap. The thermionic electron source 150 is thus lesssusceptible to ion bombardment and thus has a longer operating life thanprevious ribbon type thermionic electron sources. The life expectancy ofthe thermionic electron source depends on the operating conditions,however, for a given set of operating conditions, the life expectancy ofthe thermionic electron source is about two times greater than it wouldbe without the aperture 184. Moreover, because the improved electronsource is one piece, use of the electron source does not require supportand relative positioning of multiple emitters such as that required byan electron source having two separate and parallel emitters. Inaddition, the one piece construction may make the electron source morerigid and thus more durable and less likely to deform than an electronsource having two separate emitters.

As stated above, it is important that an electron gun to be used in anelectron beam furnace generate an electron beam suitable for the type ofmaterial to be heated and the type of deposition sought for the article.Different types of materials and depositions require electron beams ofdifferent amounts of electron beam power and may require differentelectron beam shape. It is also desirable to have the electron beam gunoperate in a space charge limited mode. However, the sixty five kilowattelectron gun operates in space charge limited mode within a power rangefrom about forty three kilowatts to about seventy kilowatts.Consequently, the sixty five kilowatt electron gun cannot adequatelygenerate electron beams for all of the electron beam powers required.

It has been determined that the accelerating electrode 22 of theelectron gun 20 can be operated with the head 26 of the electron gun 20or alternatively with the head of a second electron gun having a powerrating at least twenty five percent less than the sixty five kilowattpower rating of the electron gun 20. In this alternative, the electronbeam that results is comparable to that which would be generated by thesecond electron gun. For example, an electron gun comprising theaccelerating electrode 22 of the sixty five kilowatt gun and the head ofa forty five kilowatt gun generates an electron beam comparable to thatgenerated by the forty five kilowatt electron gun. A forty five kilowattelectron gun operates in space charge limited mode within a power rangefrom about twenty seven kilowatts to about forty eight kilowatts. Theaccelerating electrode 22 of the electron gun 20 and the head of a fortyfive kilowatt electron gun operate together in space charge limited modewithin the same range as that of the forty five kilowatt electron gun.Since the accelerating electrode 22 is not replaced and the resultingelectron gun operates at a power less than the power rating of theelectron gun 20 (sixty five kilowatts), there is no need to replace thedeflection system 38 or the cooling system 42. Therefore, suitableelectron beams of various power levels can be generated withoutreplacing the accelerating electrode 22 of the electron gun 20, thedeflection system 38 or the cooling system 42, thereby reducing theeffort involved.

Providing locating members 82, 84 that are adjustably located by atleast nine mm facilitates the operation of the accelerating electrode 22of the electron gun 20 with the head of a second electron gun that maybe smaller in size than the head 26 of the sixty-five kilowatt electrongun 20. The distance between the first locating member and the secondlocating member 84 can be sufficiently varied so as to correspond to thedistance D between the reference members of the head of the secondelectron gun. For example, as stated above, for a forty-five kilowattelectron gun, the distance between reference members on the head isforty-five mm, which is five mm (50 mm−45 mm) smaller than that of theelectron gun 20. In contrast, the prior art locating member 104A hasadjustability of less than five mm and is intended to compensate formanufacturing tolerances of components of the prior art electron gun andto facilitate alignment of the electron source and the acceleratingelectrode in the prior art electron gun. Consequently, there is lessthan desired adjustability with the prior art locating member 104A toposition a head that is five mm smaller than the head 26 of the electrongun 20.

In regard to the locating members 82, 84 and the reference members 116,118, although disclosed with respect to two locating members and tworeference members, all that is required is at least one locating memberand at least one reference member. The adjustability may be whatever isappropriate to facilitate support and positioning of heads from otherelectron guns. The spacing is preferably at least nine mm so as toaccommodate the different distances encountered between the referencemembers of the head 26 of the electron gun 20 and the reference membersof the head of an electron gun having a power rating of forty-fivekilowatts. The locating members may comprise recesses or projections.

In regard to the platform supports 44, 46, although shown with twoplatform supports, all that is required is at least one platformsupport. The platform supports need not comprise an insulator 50 and aninsulator cover 52. Any suitable type of engagement surface may be used.The platform supports need not be the same as each other. Althoughdisclosed as having a support ring 75 that engages an insulator cover52, the platform supports 44, 46 are not limited to such and may employa support member of any shape, including, but not limited to, one ormore pins.

However, it should be understood that the adjustable locating members82, 84 and the adjustable platform supports 44, 46 are not required topractice the method of the present invention. Thus, the acceleratingelectrode 22 of the electron gun 20 can be operated with the head of asecond electron gun without the presence of the adjustable locatingmembers 44, 46 or the adjustable platform supports 82, 84. For example,the head of the second electron gun may be modified to fit on theplatform, the platform may be modified in some other way to support andposition the various heads, or a plurality of different platforms may beemployed.

Although shown with one focusing electrode 152 and one acceleratingelectrode 22, there may be any number of focusing and acceleratingelectrodes. The thermionic electron source 150 need not have an aperture184. Nor must there be a sacrificial ion trap 160.

Furthermore, although described with respect to an electron gun 20having a power rating of sixty-five kilowatts, the present invention maybe used with electron guns of any power rating.

While the present invention has been described with reference to a bestmode embodiment, this description is not meant to be construed in alimiting sense. Various modifications of the best mode of embodiment, aswell as additional embodiments of the invention, will be apparent topersons skilled in the art upon reference to this description, withoutdeparting from the spirit of the invention, as recited in the claimsappended hereto. It is therefore contemplated that the appended claimswill cover any such modifications or embodiments as fall within the truescope of the invention.

What is claimed is:
 1. Apparatus for an electron gun, the apparatuscomprising; a head having a thermionic electron source that extends in alongitudinal direction and further having at least one reference member;an accelerating electrode; and a platform positioned spaced apart fromthe accelerating electrode and having at least one locating memberadjustably located on the platform, the location of the at least onelocating member being adjustably by at least nine millimeters in adirection transverse to the longitudinal direction, wherein the at leastone reference member engages the at least one locating member toposition the head in three dimensions relative to the acceleratingelectrode.
 2. The apparatus of claim 1 wherein the one of the at leastone reference member and at the least one locating member comprising aprojection, the other of the at least one reference member and the atleast one locating member comprising a recess.
 3. The apparatus of claim1 wherein the at least one reference member of the head comprising tworecesses and the at least one locating member of the platform comprisesat least two projections.
 4. The apparatus of claim 1 wherein the atleast one locating member of the platform comprising a spacer having twoholes, the platform further having crews, each of the screws having ahead and a shank, the shank having a diameter, the shank of the screwsextending through the holes and engage the platform, the holes beingelongated relative to the diameter of the shank to provide a clearancebetween the spacers and the shank to facilitate adjustment of theposition of the locating member relative to the platform and theaccelerating electrode.
 5. The apparatus of claim 1 wherein the at leastone reference member of the head comprises two recesses and the at leastone locating member of the platform comprises two locating members, eachof the locating members comprises a spacer and a projection, each of thespacers having two holes, the platform further having screws, each ofthe screws having a head and a shank, the shank having a diameter, theshank of the screws extending through the holes and engaging theplatform, the holes being elongated relative to the diameter of theshanks to provide a clearance between the spacer and the shank tofacilitate adjustment of the position of the locating members related tothe platform and the accelerating electrode.
 6. Apparatus for anelectrode gun, the apparatus comprising: a head having a thermionicelectrode source that extends in a latitudinal direction, and has atleast one reference member; an accelerating electrode; a platform thatsupports the head and having at least one locating member adjustablylocated on the platform, the location of the at least one locatingmember being adjustable by at least nine millimeters, wherein the atleast one reference member engages the at least one locating member toposition the head in three dimensions relative to the acceleratingelectrode; and at least one platform support supported by theaccelerating electrode and having a support surface that is adjustablyspaced from the accelerated electrode and supports the platform.
 7. Theapparatus of claim 6 wherein the at least one platform support comprisesan insulator, an insulator cover, and a support member, file insulatorcover is retained to insulator and has an engagement surface, and thesupport member has the support surface disposed thereon and engages theengagement surface to adjustably space the support surface from theaccelerating electrode.
 8. The apparatus of claim 7 wherein theengagement surface of the insulator cover comprises a plurality ofthreads, and the support member is a support ring with a plurality ofthreads that engage the threaded engagement surface of the insulatorcover.
 9. The apparatus of claim 8 wherein the head has at least onereference member and the platform has at least one locating memberadjustably located on the platform, the at least one locating memberhaving a location on the platform, the location of the at least onelocating member being adjustable by at least nine millimeters, whereinthe at least one reference member engages the at least one locatingmember to position the head in three dimensions relative to theaccelerating electrode.
 10. The apparatus of claim 6 wherein the atleast one reference member of the head comprises two recesses and the atleast one locating member of the platform comprises two locating member,each of the locating member comprising a spacer and a projection, eachof the spacers having two holes, the platform further having screws,each of the screws having a head and a shank, the shank having adiameter, the shank of the screws extending through the holes and engagethe platform, the holes being elongated relative to the diameter of theshank to provide a clearance between the spacer and the shank tofacilitate adjustment of the position of the locating member relative tothe platform and the accelerating electrode.
 11. The apparatus of claim8 wherein the at least one reference member of the head comprises tworecesses and the at least one locating member of the platform comprisestwo locating member, each of the locating member comprising a spacer anda projection, each of the spacers having two holes, the platform furtherhaving screws, each of the screws having a head and a shank, the shankhaving a diameter, the shank of the screws extending through the holesand engage the platform, the holes being elongated relative to thediameter of the shank to provide a clearance between the spacer and theshank to facilitate adjustment of the position of the locating memberrelative to the platform and the accelerating electrode.